Experiments were conducted to assess the influence of duration of lactation, the presence of young, and the stimulus characteristics of intruder animals upon postpartum aggression of mice. The first experiment showed that postpartum aggression toward conspecifics was highest between days 3-8, declined between days 9-14, and was present toward males but absent toward females between days 15-21 of lactation. Experiment 2 showed that lactating mice rarely attacked conspecifics to which they had been previously exposed, but would readily attack strangers. Experiment 3 and 4 demonstrated that lactating animals never attacked intruders when tested 5 hours after pup removal. However, placement of young behind a wire partition in the home-cages for 5 hours or replacement of the offspring for as little as 5 minutes following 5 hours of separation restored postpartum aggression. The 5th experiment showed that 1- and 10-day old intruders were seldom attacked while intense aggression was directed against 14- and 20-day old intruders. Finally, Experiment 6 demonstrated that 14-day old intruders whose hair was removed were rarely attacked. These results indicate that a number of factors modulate the display of postpartum aggression.